That a tiny electrical spark could bring down the world's biggest passenger plane would have been almost impossible to imagine.

But the unimaginable happened, on July 17, 1996.

Only 12 minutes after taking off for Paris from New York's John F. Kennedy International Airport, TWA Flight 800 exploded in the evening sky, showering the Atlantic off the coast of Long Island with burning fuel, pieces of the 747 jumbo jet and the bodies of all 230 people aboard.

It would take investigators four years to painstakingly piece together the clues before they could say why it blew up. Neither a bomb nor a missile -- as some initially believed -- brought down the big Boeing jet. Rather, an electrical spark of unknown origin ignited explosive fuel vapors in the center wing tank, the National Transportation Safety Board finally ruled.

More than a decade after that horrific air disaster, the Federal Aviation Administration later this year will issue its long-awaited final rules on what airplane makers and the airlines must do to better protect fuel tanks from blowing up. It's a move the industry has strongly resisted, in part because of the costs.

But even before this happens, The Boeing Co. will roll out a new jetliner in July designed with the kind of system that the safety board has said is long overdue to make sure there is no repeat of TWA Flight 800. The 787 Dreamliner will be the first new passenger jet from Boeing or Airbus that pumps inert nitrogen gas into the plane's fuel tanks as the fuel is used, to prevent the buildup of potentially explosive vapors.

"It was the right thing to do," said Mike Sinnett, Boeing's director of systems for the 787 program.

Jim Hall, an aviation lawyer who was chairman of the safety board during the investigation of TWA Flight 800, said Boeing deserves credit for its decision to have a fuel-inerting system on the 787, when much of the industry, including Airbus, has opposed the pending FAA rule.

The Airbus A380, which will replace the 747 as the world's biggest passenger plane when it enters airline service later this year, will not have a fuel-inerting system. Airbus said it is too soon to say whether it will design such a system for the A350, which is scheduled to enter service around 2013 as a competitor to the 787.

"I'm a believer that when someone in the industry is bold and takes a step for safety, they should be complimented, and Boeing should be complimented for what they have done," said Hall, a vocal and passionate advocate for fuel-tank inerting systems.

"It's taken the FAA too long to do the obvious," Hall said of the agency's final rule on fuel tank flammability, which is expected to be issued around September.

"Any rule-making process in Washington, D.C., is weighted toward the industry and toward delay," Hall said. "That's why I realized early on in my years at the NTSB that if you were going to do anything for the public good, you had to be vocal and persistent."

Fuel-tank explosions on commercial jetliners are exceedingly rare. But they have happened before -- and since -- the TWA disaster.

Until that accident, the industry generally believed that the best way to prevent fuel-tank explosions was to eliminate all possible ignition sources. But the safety board, in issuing its final report on the explosion, said that approach is "fundamentally flawed" because it is impossible to identify every potential ignition source.

The best approach, the board concluded, is to make the tanks inert, so fuel vapors can't explode.

The military has used that method for years to prevent fuel-tank explosions in planes that might be shot at in combat. But the military systems are designed to operate for only short periods of time, and they are deemed too heavy and too big for commercial jetliners.

In May 2002, the FAA unveiled the prototype of a simple on-board system that used air from the engines to produce inert nitrogen, without the need for a compressor or other moving parts. Boeing developed a similar system, which in 2005 was certified and is being evaluated on two 747-400s and two 737-700s in airline service.

Boeing said the system has performed well and will be in all factory-built 737s, 747s and 777s starting in 2008.

The FAA rule, proposed in November 2005, would require that flammability levels of vapors in the center wing tanks of more than 3,000 existing and certain new passenger jets be reduced. The rule would not mandate inerting systems, but the FAA and others agree that they are the only practical way of complying with the rule.

The industry was allowed to comment on the proposal until May 2006. The FAA got an earful.

The FAA said the proposed rule would likely cost about $808 million over 49 years, including $313 million for retrofitting the existing U.S. fleet.

But the Air Transport Association, which represents the interests of U.S. airlines and is opposed to the rule, said the costs to U.S. airlines over 49 years would be about $1.2 billion.

The inerting system that Boeing has developed will be installed on its factory-built planes next year at no cost to airlines. But putting the system on older planes will be expensive and time-consuming. Boeing said the system designed for the 737 weighs 106 to 129 pounds and its installation on 737s already in service would cost about $110,000 a plane. The weight and costs go up for bigger jets. The inerting system for the twin-aisle 777, for example, will weigh about 300 pounds and cost about $275,000 to install, according to Boeing.

In its comments to the FAA, Alaska Airlines estimated the cost to retrofit its planned fleet of jets would be about $15 million for parts and labor. That did not include lost revenue when the jets would be out of service for the retrofit, the airline said.

Like many other airlines around the world that filed comments with the FAA, Alaska argued that the FAA underestimated the costs and misrepresented the benefits.

"Alaska Airlines strongly recommends the retrofit portion (of the proposed rule) be eliminated," the airline said in its comments. But Alaska added that it does not object to the installation of a fuel-tank inerting system in new planes.

Airbus told the FAA in its comments that the agency had seriously overestimated both the risk of accidents from fuel-tank explosions and the effectiveness of the proposed safety improvements.

"The number of future accidents to passenger airplanes that might be prevented in the next 50 years by enacting this proposal is not four, as the FAA estimates, but 0.67 accidents," Airbus said.

Once the FAA rule goes into effect, it would cover both Boeing and Airbus planes, as well as those from other airplane manufacturers. Only jets registered in the United States, however, would have to comply. A foreign operator could fly a plane into the U.S. that doesn't meet the new standard.

Foreign aviation authorities often follow the FAA's lead, but European regulators are opposed to the U.S. rule.

In 2004, the NTSB wrote to European regulators urging that Airbus be required to take safety measures to prevent a possible fuel-tank explosion on the A380. The safety board noted that the FAA had requested that Airbus adopt an inerting system for the A380.

Hall, the former chairman of the NTSB, said he cannot understand the Airbus position.

"It just defies all logic that the A380, given its size, will not have a fuel-inerting system," Hall said. "I don't know whether Airbus was hung up on not wanting to do what the FAA wants them to do, and perhaps that's the problem. I fear time will reveal that was a tragic mistake."

Airbus points out that there has never been a fuel-tank explosion on one of its jetliners.

In response to written questions, Airbus said the A380 was developed "according to the latest applicable fuel tank safety rules and all efforts have been made to lower fuel tank flammability in anticipation of the forthcoming rules."

Airbus also noted that the A380 does not have a center wing tank -- the tank that blew up on the 747.

The FAA rule is expected to require that action be taken to reduce the flammability of only the center wing tanks. The problem with those tanks is that on Boeing jets, and most of those from Airbus, air-conditioning units, known as packs, are directly below the tank.

The packs give off heat that can raise the temperature of fuel vapors in the tank to potentially explosive levels. On TWA Flight 800, air conditioners under the 747's giant center wing tank ran for a couple of hours on a hot day at Kennedy airport before the plane took off. And the tank had little fuel in it, which meant it was filled with vapors.

Airbus argues that its jets are designed with a ventilation system that helps cool the heated air given off by the air-conditioning packs under the center wing tank.

But the FAA says as far as it is concerned, Boeing and Airbus planes run the same risk of a fuel-tank explosion, even though that risk may be very small.

The safety board has urged the FAA to require that the wing tanks, not just the center tank, have inerting systems.

"We believe if you can do one tank, you can do them all, and if you do them all, you eliminate the problem altogether," said Mark Rosenker, safety board chairman.

Last year, around the time of the 10th anniversary of TWA Flight 800, Rosenker complained in an interview about the industry's slow pace on fuel-tank inerting systems.

"The longer we wait, the possibility of a catastrophic explosion remains," he said then.

Hall, the former safety board chairman, recalled the 1963 crash of a Boeing 707 in a Maryland cornfield. The fuel tank on a Pan Am jetliner named the Clipper Tradewind exploded when the plane was hit by lightning. All 81 people on the plane were killed.

The crew of Flight 214 managed to send a final message -- "Clipper out of control!"

The crash was investigated by the Civil Aeronautics Board, the predecessor to the NTSB. It determined that fuel vapors in an auxiliary tank on the 707 had exploded, and in its report to the FAA, the Civil Aeronautics Board had this to say:

"It is recommended that every effort be expended to arrive at a practical means by which flammable air/vapor mixtures are eliminated from fuel tanks."